I find it really hard to believe that's the output you're getting giving the stated input and regexp. Considering each line has just a single colon, I expect the output to be:

3_
0
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If the output given is the wished output, then the answer is: there's more than just a digit and a whitespace character between your "word" and the colon. In the first line, there's ".00.00.3_3 ", the second line has ".00.04.00 ". None of them are completely matched by /[0-9]\s/. They are matched by /\.[0-9]{2}\.[0-9]{2}\.(?:[0-9]_[0-9]|[0-9]{2})\s/, or a billion other patterns.